Impaired healing and non-union are a serious complication of a fracture that results in pain, deformity, diminished function and mobility. Bone healing is a complex process which requires stem and progenitor cell migration to repair the vasculature, establish a callus and ultimately return the bone to its former shape and mechanical integrity. Delivery of bone marrow derived stem and progenitor cells to the site of injury is an effective therapy to enhance healing. Bone marrow is a rich source of endothelial progenitor cells (EPCs), mesenchymal stem cells (MSCs) and hematopoietic stem cells (HSCs), which have been shown to induce angiogenesis and osteogenesis and release anti-inflammatory cytokines to enhance bone healing. However, cell-based therapeutics such as these require the isolation of bone marrow and expansion or concentration of cells in vitro. A radically different approach is to rapidly mobilize large numbers of endogenous progenitor cells directly into the peripheral blood (PB) that will home to the site of injury and take part in tissu regeneration. AMD3100, is a selective antagonist of chemokine (CXC motif) receptor 4 (CXCR4) that rapidly mobilizes HSCs and EPCs into PB. AMD3100 in combination with VEGF mobilizes MSCs. Our central hypothesis is that bone marrow-derived progenitor cells are released into the PB subsequent to fracture, and that increasing circulating numbers of these cells using pharmacological interventions will improve bone healing. We will test this hypothesis a) by quantifying MSCs, HSCs and EPCs mobilized in response to fracture and AMD3100/VEGF in the mouse using colony forming unit assays and flow cytometry; b) by tracking the homing of mobilized progenitor cells to the fracture site; c) by measuring the effects of VEGF/AMD3100 on bone healing using a murine fracture model. We propose to mobilize cells during the first three days of fracture healing, at a time when inflammatory cytokines and chemoattractant molecules are released, and cells are thought to home to the site of injury. We anticipate that fracture results in increased numbers of MSCs, HPCs and EPCs in PB and that pharmacological intervention can significantly increase these numbers further. We anticipate that early, increased availability of circulating progenitor cells to home to the fracture site and contribute directly to tissue healing by engraftment, cell differentiation and tissue elaboration o indirectly through release of paracrine factors will accelerate bone healing. Enhanced healing will be evidenced by a greater callus size, tissue mineral density, and biomechanical strength. Our overall objective is to show that rapid release of endogenous bone marrow progenitors into PB is an effective strategy to enhance tissue regeneration. It is expected that efficient mobilization of stem cells shortly after injury, could circumvent the need for bone marrow aspiration, and complications associated with in vitro cell manipulation.